Field of the Invention
The present invention relates to a circuit configuration for driving a load. The circuit configuration contains a first connecting terminal for connecting to the load, a second connecting terminal, a first drive input for receiving a first drive signal, and a semiconductor switching element having a first load terminal connected to the first connecting terminal, a second load terminal connected to the second connecting terminal, and a drive terminal coupled to the first drive input. A voltage limiting circuit is connected between the first load terminal and the drive terminal of the semiconductor switching element.
U.S. Pat. No. 4,658,203 discloses a circuit configuration for driving a motor, which has a power MOSFET whose load path (drain-source path) is connected between a connecting terminal of the motor and ground. In this case, the gate terminal of the MOSFET is connected to a pulse width modulator for driving the MOSFET. In order to limit the voltage across the load path of the MOSFET, a reverse-biased zener diode is connected between the drain terminal and the gate terminal of the MOSFET. If the potential at the drain terminal of the MOSFET exceeds a predetermined value which is dependent on the breakdown voltage of the zener diode, then the zener diode turns on and charges the gate capacitance of the MOSFET, as a result of which the MOSFET turns on and as a result of which the voltage across the load path thereof is limited. In this case, the zener diode is dimensioned in such a way that it turns on in order to drive the MOSFET before the breakdown voltage of the transistor is reached.
For switching loads it is known, moreover, to use so-called smart power FETs. Components of this type contain, in addition to a power transistor, inter alia a protective circuit for the power transistor that is intended to protect the power transistor against, for example, an excessively large load current or an excessively high temperature. The protective circuit usually has a switch which is connected to the gate terminal of the power transistor and serves for discharging the gate capacitance of the transistor in order to turn off the transistor if, for example, the load current of the transistor or the temperature thereof assumes a value at which there is the risk of the transistor being damaged.
If a power transistor for driving a load contains a protective circuit with a switch for discharging the gate capacitance and also a zener diode for voltage limiting, then the situation can arise wherein the switch of the protective circuit turns on in order to turn off the power transistor, and wherein, at the same time, the zener diode turns on in order to drive the power transistor and thereby to limit the voltage across the load path thereof. Whereas only a short current pulse flows through the zener diode, in order to charge the gate capacitance, when the switch of the protective circuit is turned off, a current flows permanently through the zener diode when the switch of the protective circuit is turned on and the zener diode is turned on. Due to an unavoidable internal resistance of the zener diode or of another voltage limiting circuit, which may be not inconsiderable particularly in the case of integrated zener diodes, there is then the risk that, on account of the voltage drop which is additionally brought about across the zener diode and results from the product of the flowing current and the internal resistance, the drain potential of the power transistor will rise to a value at which the power transistor is in danger of destruction.
In addition, the MOSFET turns off rapidly if its gate capacitance is discharged by the protective circuit. In this case, inductive loads or else only the inductance of the leads can give rise to high induced voltages in the leads which can reach the level of the breakdown voltage of the MOSFET.
It is accordingly an object of the invention to provide a circuit configuration with a controllable current limiting circuit for driving a load which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which it is possible to use a semiconductor switching element with a protective circuit, in particular with a protective circuit against an over temperature or for current limiting, and wherein the semiconductor switching element is additionally protected against an over-voltage on its load path.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for driving a load. The circuit configuration has a first connecting terminal for connecting to the load, a second connecting terminal, a first drive input for receiving a first drive signal, and a semiconductor switching element having a first load terminal connected to the first connecting terminal, a second load terminal connected to the second connecting terminal, and a drive terminal coupled to the first drive input. A voltage limiting circuit is connected between the first load terminal and the drive terminal of the semiconductor switching element. The voltage limiting circuit has a second drive input receiving a second drive signal, and a threshold voltage of the voltage limiting circuit is dependent on the second drive signal.
The second drive signal may be variable for example depending on the switching state of the first semiconductor switching element, a temperature in the region of the first semiconductor switching element or a current through the first semiconductor switching element. The voltage limiting circuit is configured in such a way that a threshold voltage of the voltage limiting circuit is reduced in the case of those states of the first semiconductor switching element in which the voltage limiting circuit can permanently take up a current or in which it is possible for the semiconductor switch to be switched off rapidly by a protective circuit. This prevents the situation in which, as a result of a rise in the voltage across the voltage limiting circuit which results from a voltage drop brought about by a permanent current at an internal resistance of the voltage limiting circuit, the potential at the first load terminal of the first semiconductor switching element rises to a value at which the first semiconductor switching element is in danger of destruction. A situation in which the voltage limiting circuit permanently takes up current can arise when a protective circuit is present which short-circuits the drive terminal and the second load terminal of the first semiconductor switching element in the overload case, in order to turn off the first semiconductor switching element.
If the switch is in a state in which a rapid switch-off by a protective circuit can occur, then the threshold voltage of the voltage limiting circuit is preferably likewise reduced. In this case, the voltage limiting circuit counteracts the protective circuit and, upon commencement of the protective circuit, from the point when the reduced threshold voltage is reached, prevents the gate capacitance from being discharged too rapidly and thereby prevents high induced voltages on the load path of the MOSFET.
One embodiment of the circuit configuration according to the invention provides for the voltage limiting circuit to have a series circuit containing a first and a second voltage limiting element, it being possible for one of the voltage limiting elements to be short-circuited according to the second drive signal. In order to short-circuit the one voltage limiting element, in one embodiment of the invention, a switch, in particular a semiconductor switching element, is connected in parallel with the voltage limiting element, the switch having a drive terminal to which the second drive signal is fed. The series-connected voltage limiting elements are preferably zener diodes, which have the advantage that they can be integrated in a simple manner in the same semiconductor body as the first semiconductor switching element.
Depending on the switch position of the second switch, the voltage limiting circuit according to the invention has a breakdown voltage that corresponds to the sum of the breakdown voltages of the two voltage limiting elements or the breakdown voltage of the voltage limiting element which cannot be short-circuited.
In one embodiment of the invention, the second drive signal is dependent on a switching state of the first semiconductor switching element. As a result, a high breakdown voltage of the voltage limiting circuit can be set in the case of a first switching state, in which the first semiconductor switching element turns off, whereas a lower breakdown voltage of the voltage limiting circuit is set in the case of a second switching state, in which the first semiconductor switching element turns on. In the case of the switching state mentioned last, given the presence of a protective circuit which serves for turning off the first semiconductor switching element in an overload case, the situation can arise wherein a current permanently flows through the voltage limiting circuit, since part of the current through the voltage limiting circuit which is intended to turn on the first semiconductor switching element is taken up by the protective circuit. The protective circuit is used to achieve an opposite aim, namely to turn off the first semiconductor switching element. Due to an internal resistance that is inevitably present in the voltage limiting circuit, in the event of a permanent current flow through the voltage limiting circuit, the voltage across the voltage limiting circuit rises. The voltage limiting circuit has a breakdown voltage that is reduced in this case, however, and this prevents the voltage across the load path of the first semiconductor switching element from rising to a value at which the first semiconductor switching element is in danger of destruction.
One embodiment of the invention provides a protective circuit with a third semiconductor switching element, the third semiconductor switching element is connected between the control terminal of the first semiconductor switching element and the second load path terminal thereof. The third semiconductor switching element serves for turning off the first semiconductor switching element according to a drive signal, in that it short-circuits the drive terminal and the second load path terminal. In this case, the drive signal of the third semiconductor switching element is preferably dependent on a temperature in the region of the first semiconductor switching element and/or on a load current of the first semiconductor switching element.
A second embodiment of the invention provides for the second drive signal to be dependent on a temperature in the region of the first semiconductor switching element. As a result, the breakdown voltage of the voltage limiting circuit can be reduced for example when the temperature in the region of the first semiconductor switching element exceeds a predetermined value at which a protective circuit reacts in order to turn off the first semiconductor switching element. In this case, too, the protective circuit pursues an opposite aim to that of the voltage limiting circuit, namely the aim of turning off the first semiconductor switching element, which results in a permanent current flow through the voltage limiting circuit. The reduced breakdown voltage of the voltage limiting circuit in this case prevents the breakdown voltage of the first semiconductor switching element from being reached, the first semiconductor element being in danger of destruction at the breakdown voltage.
In a corresponding manner, a further embodiment of the invention provides for the second drive signal to be dependent on a current through the first semiconductor switching element. In this case, if the current through the first semiconductor switching element exceeds a value at which a protective circuit switches on for the purpose of turning off the first semiconductor switching element, then, as in the over-temperature case described above, the breakdown voltage of the voltage limiting circuit is reduced in order to intercept the voltage rise across the voltage limiting circuit, the voltage rise being brought about in the event of a permanent current flow, and to prevent the breakdown voltage of the first semiconductor switching element from being reached.
The voltage limiting circuit preferably has a drive circuit for driving the second switch connected in parallel with the first voltage limiting element, the drive circuit being fed a current signal which is dependent on the current through the first semiconductor switching element, and/or a temperature signal which is dependent on the temperature in the region of the first semiconductor switching element. In this case, for the provision of the current signal, a current sensor is connected in series with the first semiconductor switching element and, for the provision of the temperature signal, a temperature sensor is disposed in the region of the first semiconductor switching element.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration with a controllable current limiting circuit for driving a load, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.